Method for reducing the temperature of bakery products

ABSTRACT

The temperature of bakery products is reduced immediately after the discharge thereof from the oven, by means of vacuum, by first subjecting the bakery products to a substantially linear pressure reduction as a function of time at a rate of not more than 60 mm Hg per second through at least half of the total pressure drop to be achieved. The pressure reduction rate is then allowed to increase substantially for a terminating vacuum treatment. At least one vacuum chamber is provided and is adapted to be connected to a source of vacuum through a vacuum conduit and a valve, and has a closeable opening for bringing bakery products into and out of the chamber. In the vacuum conduit, in parallel to a servo-valve serving to apply a full vacuum effect from the source of vacuum during the remaining portion of the total pressure drop, there is inserted a by-pass conduit having a permanently open cross-section which can be adjusted to provide the substantially linear pressure reduction.

This is a continuation-in-part of application Ser. No. 541,944, filedJan. 17, 1975, now abandoned.

BACKGROUND OF THE INVENTION

This invention generally relates to a method and apparatus for reducingthe temperature of bakery products immediately after the dischargethereof from the baking oven.

A particularly interesting form of bakery products in this connection isbread. The last stage of the baking process of a loaf of bread takesplace in an oven having a temperature of from 200° to 270° C. with abaking time which varies from 45 minutes to 20 minutes, depending uponthe temperature and the cross-section of the loaf of bread. The crumb ofthe loaf has a temperature of about 95° C. at the termination of thebaking time, whereas the temperature of the crust is about 150° C., theformer temperature having been rather constant during the last part ofthe baking time because of evaporation of the moisture content in theloaf of bread.

For a number of different reasons it has been common practice to wrapbakery products, in particular bread, in bags for distribution and sale.

If the loaves are packed directly from the oven, the bag cannot beclosed since the vapour from the interior of the loaf cannot find anypath of escape and condenses on the inner side of the bag. This makesthe bag soft. If a bag of plastic material is used, the surface of theloaf of bread will be softened by the water resulting from the condensedvapour. In the case of bread for which a crisp crust is a qualitycriterion (wheat bread), care should be taken during packing of the warmloaves that the open bag with its content is put in an upright positionso that easy vapour escape is secured. The use of plastic materials forpacking bread types having a long shelf life or good keeping qualities(whole meal bread) is not possible unless the loaves are first cooled.Bakers who have found it advantageous to sell whole meal bread in closedbags of plastic material therefore must provide for the cooling of theloaves of bread at least for one hour before packing, either by loadingthem on shelf carriages which are rolled into large cooling halls, or bymeans of a conveyor system which takes the loaves on a round triplasting an hour or so. Such arrangements are very labour-consumingand/or require large investments.

Cooling of the loaves of bread in the above arrangement takes place as acombination of (a) radiation, (b) convection and (c) evaporation. If ahigh relative humidity is maintained in the cooling space, theevaporation rate will be lower, which means that the loss of breadweight is reduced, which of course, is of importance. The reduced loss,however, must be seen in relation to the expenses in the form of labour,time and capital which are necessary in order to obtain this result.

It is known that cooling of bakery products can be effected by means ofvacuum treatment, which is described in "The Bakers Digest", Oct. 1949and in "The Arkady Review", Volume 49, No. 2, 1973. The previouslydescribed methods for the vacuum cooling of bread, however, have notbeen completely successful and in particular have not made it possibleto obtain a maximum of processing capacity with reasonable installationand operation expenses for the necessary equipment. An importantrequirement of the method is also that the vacuum treatment must notresult in damage to the structure of the bakery product, in particularthe surface or crust thereof. This represents a serious problem whichfor the first time has been solved with the present invention insofar aslarge capacity cooling is concerned.

SUMMARY OF THE INVENTION

As in many mass production processes, the key to an economic andeffective production is dependent upon the time period of treatment ineach process step being reduced to a minimum. For the vacuum treatmentof bakery products a vacuum chamber is necessarily required, and it isan essential prerequisite to this invention that the vacuum chambersemployed have a comparatively small volume. This makes it possible witha moderate pumping capacity in the vacuum system to apply shorttreatment times in these chambers, whereby the operating rate and thusthe production capacity will be high.

According to the invention it has been found that a short vacuumtreatment time makes it necessary to keep close supervision and controlof the pressure variation in the chamber. Thus, according to theinvention it is essential that the bakery products are first subjectedto a substantially linear pressure reduction at a velocity of not morethan 60 mm Hg per second, at least through half of the total pressuredrop employed, and that the pressure reducing velocity is then allowedto increase substantially for a finishing vacuum treatment.

The bakery products are subjected to the first linear pressure reductionimmediately after discharge from the oven and without subjecting thebakery products to an initial air cooling operation.

A desired loaf temperature can be obtained by putting the loaf in achamber in which the pressure is reduced to a value corresponding to thevapour pressure at the desired temperature. Water having a highertemperature than the desired temperature will boil at this pressure. Theheat bound by the boiling is taken from the loaf. In practice thecooling follows the rate of pressure reduction. As mentioned above,however, it is shown in connection with this invention that there is apractical limit to the pressure reducing velocity. If the velocity istoo high, damage will be caused to the loaf because of the relativelylarge amounts of vapour released.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description the invention shall be explained moreclosely with reference to the attached drawings in which:

FIG. 1 is a diagram illustrating the pressure variation during oneexample of a vacuum treatment operation according to the invention, and

FIG. 2 is a schematic view of a vacuum chamber with associated equipmentfor cooling bakery products, in particular bread.

DETAILED DESCRIPTION OF THE INVENTION

The pressure reducing process shown in FIG. 1 takes place in two stagesfrom a point A corresponding to atmospheric pressure. The first stageruns substantially linearly from the starting point at A to a point Bduring about 12 seconds, and during which about 60% of the totalpressure drop employed is traversed, i.e. from about 760 to about 373 mmHg. This linear pressure drop can adequately be provided for by means ofpressure equalization through a conduit having a cross-sectionalrestriction, for instance a nozzle of about 5 mm diameter. This devicewill be described in more detail below. Whereas during the firstpressure reduction stage between the points A and B it is of greatimportance that the rate of pressure reduction does not exceed a certainvalue, depending among other things to a certain degree upon the type ofbakery products being treated, the remaining portion of the pressuredrop can be effected without such limitation. This appears from thecurve of FIG. 1, which from the point B falls off with substantiallygreater inclination than during the first portion of the pressurereduction. As usual in such pressure equalization processes the curvewill eventually flatten out, and the treatment according to theinvention is terminated at a point C after a total treatment time ofabout 20 seconds, somewhat more than 80% of the original pressure havingthen been removed. The terminating pressure will correspond to thevapour pressure at the desired temperature after the vacuum treatment.The values shown in the diagram of FIG. 1 relate to a temperaturereduction to about 54° C. corresponding to a vapour pressure of about0.15 kp/cm² or 85% vacuum. This corresponds to 115 mm Hg in a normalatmosphere. When treating seven loaves of bread simultaneously (asindicated by way of example in FIG. 2), the following figures can serveto illustrate the method of the invention. After baking, the weight ofseven loaves is about 6.25 kg, of which about 37% is water. At fullbalance between the vapour pressure of the water content and the ambientatmosphere the evaporated water will correspond to the cooling of theloaves from 95° C. to 54° C.

Half or preferably 60% of the pressure drop is linear in order that thevapour developed in the loaf of bread will be able to escape through thenatural pores of the crust at a pressure difference which does not breakthe crust. The pressure reducing velocity is lower than 60 mm Hg persecond, which in the pressure reducing process example of FIG. 1corresponds to performing the linear portion of the pressure reduction,i.e. from 760 to 373 mm Hg, during not less than approximately 6.5seconds. It should be noted that line A--B in the example of FIG. 1illustrates a linear pressure drop of approximately 32 mm Hg/second,i.e. at a rate less than the maximum rate. The terminating pressure andthe linear portion of the process are variable.

In the example shown in FIG. 1 the total pressure is reduced from normalatmospheric pressure, i.e. 760 mm Hg, to 115 mm Hg. This corresponds toan approximately 85% vacuum reduction. In this specific example thetemperature is reduced to approximately 54° C. It is to be understoodthat the scope of the present invention includes reductions to pressurescorresponding to other temperatures. For example, it may be desirable inthe bakery product cooling operation to reduce the temperature down 35°C., which temperature would correspond to a pressure reduction toapproximately 42 mm Hg. Further, it may be sufficient in othercircumstances to reduce the temperature of the bakery products toapproximately 60° C., such temperature corresponding to a pressurereduction to approximately 150 mm Hg.

Accordingly, it will be understood that the overall temperaturereduction is chosen for a particular bakery product or environment, andthen the parameters of the two pressure reduction operations aredesigned.

It is important in accordance with the present invention that thepressure drop during the first pressure reduction operation correspondto at least half of the total pressure reduction of the first and secondpressure reduction operations.

It is further important in accordance with the present invention thatthe maximum rate of pressure reduction in the first linear pressurereduction operation be lower than 60 mm Hg per second. The minimumvelocity of pressure rate reduction is not particularly critical. It isof course desired that the pressure reduction, and therefore thecooling, be achieved as rapidly as possible. The actual linear pressurereduction rate of the first pressure reduction operation employed willdepend upon certain practical considerations, such as for examplecooling equipment capacity. It has however been found that a minimumpressure reduction rate for the first or linear pressure reductionoperation of 10 mm Hg per second is satisfactory. This minimum rate isof course substantially slower than the maximum allowed rate, but isintended to be within the scope of the present invention, as long as theother parameters and requirements of the present invention are followed.A further possible minimum rate of pressure reduction to be employedduring the first or linear pressure reduction operation may be 30 mm Hgper second.

However, as discussed above, the critical feature of the presentinvention is that the maximum rate of pressure reduction during thefirst or linear pressure reduction operation not exceed 60 mm Hg persecond.

A schematic and simplified example of an apparatus according to theinvention is shown in FIG. 2. This apparatus is based on cooperationwith an oven from which the loaves of bread are pushed out in groups orrows of 7-8 loaves in a direction parallel or transverse to thelongitudinal direction of the oven at an angle of 90° by means of aconveyor or an ejector associated with the oven. The loaves 2 areconveyed in their longitudinal direction into a cylindrical vacuumchamber 1 which can be closed with a cover 3 to which there is connecteda vacuum conduit 4 with associated valves and connections. Morespecifically, this vacuum system consists of a branch piece 10, a shortvacuum conduit 9 and a servo-valve 6 which through an additional vacuumconduit 5 serves to connect the vacuum chamber 1 to a vacuum pump oranother source of vacuum which is not shown in the drawings. Parallel tothe servo-valve 6 there is provided a by-pass conduit 8 which has acalibrated or adjustable cross-section which is restricted in relationto the open flow cross-section of servo-valve 6, so as to give theslower, near to linear pressure reduction at the beginning of the vacuumtreatment. For adjustment or control of the linear pressure reductionrate there is inserted a control valve 7 in the by-pass conduit 8.

For terminating the vacuum treatment a magnet valve 12 is connected tothe branch piece 10 through a conduit 11 for allowing atmosphericpressure to enter the chamber 1. For this purpose the magnet valve 12 isconnected to the ambient atmosphere through a conduit 13.

Especially during starting up of the process the vapour from the bakeryproducts may condense in the cylindrical chamber. In practice it maytherefore be an advantage to provide means for preventing condensedwater from reaching the crust on the products being treated. Forhygienic reasons, however, it may be desirable to avoid inlays, insertsor the like in the chamber for this purpose. Condensation is preventedby heating the chamber. This can be done by suitable means, for instanceby radiant heat.

In connection with the drawings there is no detailed illustration ofmechanisms for opening and closing the cover 3 on the chamber 1 in FIG.2, but the necessary structure for obtaining these functions will bereadily understood by persons skilled in the art.

Various modifications may be made to the above described specificstructural and operational features without departing from the scope ofthe invention.

What is claimed is:
 1. A process for rapid vacuum cooling of hot bakeryproducts, immediately after the discharge thereof from an oven, to atemperature suitable for packaging said bakery products, and for formingsaid bakery products with a relatively thick and crisp crust withoutbursting said crust, said process consisting essentially of:subjectingbakery products, immediately after the completion of baking anddischarge thereof from an oven at atmospheric pressure, and withoutsubjecting said bakery products to a deliberate initial air coolingoperation, to a first pressure reduction which is substantially linearas a function of time at a rate of from 10 to 60 mm Hg per second,thereby preventing bursting of the crust of said bakery products; andimmediately thereafter subjecting said bakery products to a secondnon-linear pressure reduction at an increased rate, and reducing thepressure to a value corresponding to the vapor pressure of a desiredtemperature suitable for packaging said bakery products; the reductionof pressure during said first pressure reduction comprising at leasthalf the total reduction of pressure of said first and second pressurereductions; and wherein substantially all of the cooling of the bakeryproducts is done by using a vacuum to subject the bakery products toreduced pressures.
 2. A process as claimed in claim 1, wherein saidreduction of pressure during said first pressure reduction comprisesapproximately 60% of said total pressure reduction.
 3. A process asclaimed in claim 2, wherein said first pressure reduction is completedin approximately 12 seconds, and said total pressure reduction iscompleted in approximately 20 seconds.
 4. A process as claimed in claim1, wherein both said first and second pressure reductions are achievedby use of a single vacuum source.